Sunday, March 4, 2012

707. Thickest Parts of Arctic Ice Cap Melting Faster

How perennial sea ice has declined from 1980 to 2012. The bright white central mass shows the perennial sea ice while the larger light blue area shows the full extent of the winter sea ice including the average annual sea ice during the months of November, December and January. (Credit: NASA/Goddard Scientific Visualization Studio)

A new NASA study
revealed that the oldest and thickest Arctic sea ice is disappearing at a
faster rate than the younger and thinner ice at the edges of the Arctic Ocean's
floating ice cap.

The thicker ice,
known as multi-year ice, survives through the cyclical summer melt season, when
young ice that has formed over winter just as quickly melts again. The rapid
disappearance of older ice makes Arctic sea ice even more vulnerable to further
decline in the summer, said Joey Comiso, senior scientist at NASA Goddard Space
Flight Center, Greenbelt, Md., and author of the study, which was recently
published in Journal of Climate.

The new research
takes a closer look at how multi-year ice, ice that has made it through at
least two summers, has diminished with each passing winter over the last three
decades. Multi-year ice "extent" -- which includes all areas of the
Arctic Ocean where multi-year ice covers at least 15 percent of the ocean
surface -- is diminishing at a rate of -15.1 percent per decade, the study
found.

There's another
measurement that allows researchers to analyze how the ice cap evolves:
multi-year ice "area," which discards areas of open water among ice
floes and focuses exclusively on the regions of the Arctic Ocean that are
completely covered by multi-year ice. Sea ice area is always smaller than sea
ice extent, and it gives scientists the information needed to estimate the
total volume of ice in the Arctic Ocean. Comiso found that multi-year ice area
is shrinking even faster than multi-year ice extent, by -17.2 percent per
decade.

"The average
thickness of the Arctic sea ice cover is declining because it is rapidly losing
its thick component, the multi-year ice. At the same time, the surface
temperature in the Arctic is going up, which results in a shorter ice-forming
season," Comiso said. "It would take a persistent cold spell for most
multi-year sea ice and other ice types to grow thick enough in the winter to survive
the summer melt season and reverse the trend."

Scientists
differentiate multi-year ice from both seasonal ice, which comes and goes each
year, and "perennial" ice, defined as all ice that has survived at
least one summer. In other words: all multi-year ice is perennial ice, but not
all perennial ice is multi-year ice (it can also be second-year ice).

Comiso found that
perennial ice extent is shrinking at a rate of -12.2 percent per decade, while
its area is declining at a rate of -13.5 percent per decade. These numbers
indicate that the thickest ice, multiyear-ice, is declining faster than the
other perennial ice that surrounds it.

As perennial ice
retreated in the last three decades, it opened up new areas of the Arctic Ocean
that could then be covered by seasonal ice in the winter. A larger volume of
younger ice meant that a larger portion of it made it through the summer and
was available to form second-year ice. This is likely the reason why the
perennial ice cover, which includes second year ice, is not declining as
rapidly as the multiyear ice cover, Comiso said.

Multi-year sea ice
hit its record minimum extent in the winter of 2008. That is when it was
reduced to about 55 percent of its average extent since the late 1970s, when
satellite measurements of the ice cap began. Multi-year sea ice then recovered
slightly in the three following years, ultimately reaching an extent 34 percent
larger than in 2008, but it dipped again in winter of 2012, to its second
lowest extent ever.

For this study,
Comiso created a time series of multi-year ice using 32 years of passive
microwave data from NASA's Nimbus-7 satellite and the U.S. Department of
Defense's Defense Meteorological Satellite Program, taken during the winter
months from 1978 to 2011. This is the most robust and longest satellite dataset
of Arctic sea ice extent data to date, Comiso said.

Younger ice, made
from recently frozen ocean waters, is saltier than multi-year ice, which has
had more time to drain its salts. The salt content in first- and second-year
ice gives them different electrical properties than multi-year ice: In winter,
when the surface of the sea ice is cold and dry, the microwave emissivity of
multiyear ice is distinctly different from that of first- and second-year ice.
Microwave radiometers on satellites pick up these differences in emissivity,
which are observed as variations in brightness temperature for the different
types of ice. The "brightness" data are used in an algorithm to
discriminate multiyear ice from other types of ice.

Comiso
compared the evolution of the extent and area of multi-year ice over time, and
confirmed that its decline has accelerated during the last decade, in part
because of the dramatic decreases of 2008 and 2012. He also detected a periodic
nine-year cycle, where sea ice extent would first grow for a few years, and
then shrink until the cycle started again. This cycle is reminiscent of one
occurring on the opposite pole, known as the Antarctic Circumpolar Wave, which
has been related to the El Niño-Southern Oscillation atmospheric pattern. If
the nine-year Arctic cycle were to be confirmed, it might explain the slight
recovery of the sea ice cover in the three years after it hit its historical
minimum in 2008, Comiso said.